There are a lot of myths about synthetic and conventional oil. Synthetic is much better for the engine in my opinion, it does not breakdown from heat as fast as conventional oil and works well under different ranges of temperature. Also depending on the brand/type different additives is added to the oil which can also help clean,prevent and protect engine from sludge build up. You can gather more info on this from manufacturers websites like Castrol, Valvoline, Mobile1, AMSOIL, Royal purple etc...

With a true FULL synthetic you do not play chemists by adding any additives. There is no need.
A real full synthetic lubricant stands only by it self.
With a true full synthetic lubricant there is no, I repeat no sludge build up.